1. Field of the Invention
The present invention relates to a nibbler assembly for a punch press to perform a nibbling operation performed on a sheet material, a method of forming an elongated hole in the sheet material with such a nibbler assembly, and a punch for a punch press to perform a nibbling operation to the sheet material.
2. Description of the Related Art
In a nibbling operation performed on a sheet material with a punch press, a conventional process may include the use of a die and a punch to punch a plurality of successive holes that at least partially overlap with each other. The die may include a through opening formed therein and having an oblong configuration. Such a conventional process may leave a seam-like portion at one end of the overlap between the successive holes, which may deteriorate an appearance or may cause an uncomfortable touch. Such a seam-like portion consists of the following components as shown in FIG. 13.
Assuming that a punch of a laterally rectangular configuration is used to punch a hole 51, a roll-over or shear droop may not be formed at the upper end edge 51aa of the corner 51a of the punched hole 51, as shown in FIG. 14. This results in the formation of a seam-like segment 56 on a roll-over surface 53. More specifically, the upper end edge 51aa of the corner 51a with no roll-over leaves a projection when the next hole 51 is punched. It has been discovered that the absence of roll-over at the upper end edge 51aa of the corner 51a is caused by the greater distance between a punch 1 and the punched hole 51 at the corner 51a than the distance C at other locations (B>C), as is illustrated in FIG. 15. This greater distance at the corner 51a makes it difficult to impart the punching force of the punch 1 to a sheet material W.
Also, at the corner 51a of the punched hole 51, the punched surface is not made of a fractured surface. Instead, as shown in FIG. 14, a burnished or sheared surface segment 54a continues to the lower end of the punched surface, causing the formation of a protrusion 57 on the punched surface. Note that the fractured surface 55 is depressed with respect to a sheared surface 54, as is illustrated in FIG. 16. Therefore, of the sheared surface segment 54a left in the previously punched hole 51, the portion that extends to the lower end remains in the form of a projection protruding from the surrounding fractured surfaces, when the next hole 51 is punched.
Moreover, since at the corner 51a of the punched hole 51 previously punched in the sheet material W, the sheared surface segment 54a (FIG. 14) extends to the lower end of the sheet material W, the sheared surface segment 54a is rubbed and stretched downwardly by a side surface of the punch 1 when it punches the next hole 51. This results in the formation of a prong 58 that protrudes from the bottom surface of the sheet material W. The prong 58 is a burr of a larger size.
Conventionally, these seam-like segments are manually treated, for example, with a sander or a file. Manual treatments not only lead to a variation in the finishing quality due to the skill of attendant workers but also involve more process steps, thereby lowering the process efficiency.
In order to eliminate the formation of such seam-like segments of various kinds or make them as invisible as possible, a punching technique has been proposed which uses a nibbler assembly with a cutter blade section having a bottom surface slant with respect to a horizontal axis. The slant cutter blade section is not configured to punch a complete hole in a sheet material, but is only configured to make small slits that facilitate a complete cutting in the next punching stroke (see, for example, Japanese Patent No. 3401908 and Japanese Patent No. 3960066). Japanese Patent No. 4518539 proposes a punching technique which makes successive half-cuts in a workpiece at a predetermined interval and subsequently makes a complete cut in a portion of the workpiece extending between the neighboring half-cut points, in order to eliminate seam-like portions. Japanese Examined Patent Publication No. S61-009895 describes a punch with a side edge having an arcuate shape in order to perform a nibbling operation in such a way that the arcuate shape portion of the punch overlaps with the portion of a previously formed hole which does not contain any arcuate shapes, in order to make seam-like segments on a roll-over surface less visible.
The above proposed techniques enable the formation of roll-over or shear droop and fractured surfaces at each corner of a punched hole, just as in other locations of the punched hole. With such techniques, a nibbling operation to form successive holes with a partial overlap with each other allows for aesthetic finishing of the overlapped portion between successive holes, resulting in smooth and continuous surface textures. However, the kind of punch used in these techniques has a complex shape and is accordingly expensive. Also, since the sheet material is not punched out by the slant cutter blade section, a hole that can be punched per stroke is of a limited length and accordingly, the process efficiency is low. Furthermore, complicated control must be implemented to assure a fine adjustment of the stroke length of the punch.